Does using base64 vs UTF-8 HTML/Text impact email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 21 Jul 2025
Updated 15 Aug 2025
9 min read
The question of whether using Base64 versus UTF-8 HTML/Text impacts email deliverability is a common one, and it often arises from a misunderstanding of what these terms actually represent in the context of email. They are not directly interchangeable methods for encoding an email's primary content, but rather serve different, complementary purposes in preparing an email for transmission.
To properly understand their roles, we need to differentiate between character encoding and content-transfer encoding. Character encoding, such as UTF-8, dictates how human-readable characters are converted into a sequence of bytes. Content-transfer encoding, on the other hand, deals with how those bytes (or any other binary data) are prepared to be safely transported across an email system, which historically was designed only for 7-bit ASCII text.
The difference between character and content transfer encodings
When sending an email, both a character encoding and a content-transfer encoding are always necessary. Character encoding, specified by the charset parameter in the Content-Type header, defines how the text itself is represented. UTF-8 is the universally recommended standard for email character encoding, as it supports almost all characters and symbols across different languages. This is crucial for global communications and ensures your message displays correctly for all recipients.
Content-transfer encoding, on the other hand, determines how the encoded text, or any binary data like images, is converted into a format suitable for transmission over email protocols. The two most common methods are Base64 and Quoted-Printable. These encodings allow non-ASCII characters or binary data to be safely transmitted through systems that might otherwise corrupt them. They effectively armor data so it adheres to the 7-bit ASCII standard of email transport.
Choosing the correct content-transfer encoding is important, not only for technical compliance but also for subtle deliverability implications. For instance, using UTF-8 as the character set ensures broad compatibility, while the choice between Base64 and Quoted-Printable for the content-transfer encoding impacts factors such as email size and how readily human reviewers can inspect the email's content without specialized tools.
Encoding Type
Purpose
Best Use Case
Impact on Size
UTF-8
Character encoding: Converts characters into bytes for digital storage and transmission.
Universal text content, international characters, emojis.
Efficient for ASCII, slightly larger for non-ASCII characters.
Base64
Content-transfer encoding: Converts binary data into a text format for safe email transmission.
Binary data such as images, attachments, or encrypted content.
Increases size by approximately 33% compared to original binary data.
Quoted-Printable
Content-transfer encoding: Converts 8-bit text data into a 7-bit ASCII format.
Text-heavy emails, especially those with some non-ASCII characters.
Minimal size increase, only for non-ASCII characters, maintains readability.
How base64 encoding affects email deliverability
In the past, Base64 encoding for plain text or HTML email content did, at times, carry a negative connotation for deliverability. This was because spammers would frequently use Base64 encoding as a tactic to obfuscate their malicious content and bypass rudimentary content filters. However, this strategy quickly became ineffective as email security systems evolved. Modern spam filters are highly sophisticated and can easily decode Base64 content to analyze the underlying text, rendering this evasion technique obsolete.
While the direct deliverability impact from Base64 encoding of text is minimal today, there are still practical considerations. Base64 encoding always increases the size of the data by approximately 33%. For text-heavy emails, this can contribute to a larger overall email file size. Larger email sizes can sometimes lead to slower transmission, increased bandwidth usage, and may occasionally trigger file size limits or scrutiny from spam filters, though this is less common for text than for large embedded binary data, like images. For more on email file size, see our guide on how email file size affects deliverability.
Another consideration, particularly when dealing with plain text (or text/html) parts of an email, is the potential for human review. While automated systems will decode Base64 without issue, if an email ever lands in a situation where a human analyst (perhaps at an internet service provider or security firm) needs to inspect the raw message source, a Base64 encoded text part will be unreadable to the naked eye. This lack of immediate readability, while not a direct deliverability blocker, could subtly impact how the email is perceived, potentially raising a flag if suspicion already exists. This is why some advocate that Base64 encoding of plain text content is generally an unnecessary practice. We explore this further in our article on how Base64 encoding of plain text impacts spam filter scores.
Historical spam evasion
In earlier days of email, spammers would often Base64 encode their plain text content to bypass basic keyword and content filters, as these filters weren't equipped to decode it. This led to a historical association of Base64-encoded text with suspicious activity.
The result was that email providers (ISPs) often assigned negative reputation scores to messages with Base64-encoded plain text or HTML, even if the content itself wasn't inherently spammy. This was a simplistic, but effective, measure at the time to combat unwanted mail.
Why UTF-8 and quoted-printable are preferred for text
For email bodies that are primarily text, especially those that include non-ASCII characters like accented letters, symbols, or characters from non-Latin alphabets, Quoted-Printable encoding is generally the superior choice for content-transfer encoding. Quoted-Printable encodes only the characters that strictly require it (i.e., those outside the 7-bit ASCII range), leaving the majority of plain text readable. This maintains a balance between ensuring safe transmission and keeping the message source legible. Our article on multiple language email implications offers more insight into this.
Conversely, Base64 encoding is specifically designed for arbitrary binary data. This makes it ideal for embedding images within an email, attaching files, or transmitting encrypted content, where readability of the raw encoded stream is not a concern. In these scenarios, the size increase from Base64 is acceptable because there's no more efficient way to send binary data via email without the risk of corruption. You can learn more about how images affect deliverability and embedding images with Base64.
For email clients and servers, the exact content-transfer encoding used for text often doesn't make a significant difference in terms of deliverability, as long as it's correctly applied and compatible. However, adhering to the best practices for each encoding type helps ensure maximum compatibility and can prevent unnecessary flags from security systems that might still have historical biases. A well-formed email, including proper encoding, is a fundamental step towards good inbox placement.
Recommended content-type header for HTML emails with UTF-8 and Quoted-Printable
The good news is that for most senders, the choice of content-transfer encoding for plain text or HTML content generally has minimal direct impact on email deliverability today. Modern spam filters and receiving mail servers have become extremely adept at processing and decoding various encoding schemes, including Base64. They no longer rely on simplistic checks, like looking for Base64 encoded text, to make critical spam decisions. Their algorithms are far more sophisticated, focusing on a multitude of factors, including sender reputation, content analysis, authentication records like why your emails fail, and user engagement.
The primary decisions about content-transfer encoding are typically handled at a high level by your Mail Transfer Agent (MTA) or email service provider (ESP), not directly by the person crafting the email's content. These systems are designed to choose the most appropriate encoding based on the email's content type (text or binary) and character set. As a sender, while you might not directly control this, understanding the implications can help you advocate for best practices with your technical teams.
While automated systems are intelligent, there might be rare instances where a human spam analyst or security expert examines the raw source of an email. In such edge cases, an email with readily viewable content, such as that encoded with Quoted-Printable, might foster a perception of transparency and goodwill. This small factor, while not a primary deliverability driver, can contribute positively to overall trust signals. For more information on ensuring your emails bypass spam filters, check out our guide on boosting email deliverability rates.
Views from the trenches
Best practices
Always use UTF-8 as your character encoding for email content to ensure broad compatibility and correct display of diverse characters and languages.
Utilize Quoted-Printable for content-transfer encoding when your email is primarily text, especially if it contains non-ASCII characters, as it maintains readability and efficiency.
Employ Base64 encoding exclusively for binary data within emails, such as embedded images or attachments, as this is its intended and most efficient use.
Ensure your email sending platform or MTA is configured to automatically apply the appropriate character and content-transfer encodings based on the email's content.
Focus on overall email deliverability best practices, including sender reputation, authentication, and content quality, as encoding is just one small part of the deliverability puzzle.
Common pitfalls
Base64 encoding plain text or HTML content unnecessarily increases email size by approximately 33%, potentially leading to slower loading times and unnecessary bandwidth consumption.
Using Base64 for text content makes the raw email source unreadable to human reviewers, which historically raised spam flags and could still subtly impact perception in edge cases.
Failing to specify UTF-8 as the character set can lead to rendering issues for recipients, causing garbled text or incorrect display of international characters.
Incorrectly mixing or omitting character and content-transfer encodings can result in broken emails that are unreadable or flagged by spam filters due to malformed MIME parts.
Relying on outdated beliefs that Base64 encoding can bypass modern spam filters, which are now highly sophisticated and adept at decoding all standard encodings.
Expert tips
For text content, Quoted-Printable is often superior because it only encodes necessary characters, keeping the raw message more readable and smaller. Base64 is ideal for binary data like images.
Modern email filters are highly advanced and can easily decode both Base64 and Quoted-Printable, so direct deliverability impact from the choice of encoding for text is minimal.
While encoding decisions are mostly handled at the MTA level, it's beneficial to understand these differences to communicate effectively with developers or email service providers.
In rare cases where a human reviews email source code, a Quoted-Printable encoded message is much easier to read, which can contribute positively to a perception of legitimacy.
Always ensure your entire email infrastructure, from content creation to sending, correctly implements both character and content-transfer encodings to prevent deliverability issues.
Expert view
Expert from Email Geeks says that character encodings and content-transfer encodings are distinct concepts, which can often be confused.
2020-11-25 - Email Geeks
Expert view
Expert from Email Geeks notes that years ago, Base64 encoding for plain text or HTML often led to negative sender reputation because spammers used it to bypass early content filters, though filters now easily decode such content.
2020-11-25 - Email Geeks
Choosing the right encoding for your emails
While the fundamental deliverability impact of choosing Base64 versus Quoted-Printable for text content has diminished over time due to advancements in spam filtering technology, the best practice remains clear. For email bodies (HTML or plain text) that are predominantly textual, Quoted-Printable is the preferred content-transfer encoding. It efficiently handles non-ASCII characters while keeping the message source more readable, which can be beneficial in rare human review scenarios. Concurrently, always use UTF-8 as your character encoding for universal compatibility.
For binary data, such as images embedded in your email or attachments, Base64 remains the appropriate and necessary content-transfer encoding. Your email sending platform should ideally manage these encoding decisions automatically, ensuring that the correct methods are applied for different parts of your message. By adhering to these well-established standards, you optimize email rendering for recipients globally and maintain a transparent, trustworthy sending posture, contributing to overall email deliverability health. You can see our comprehensive guide on email deliverability issues for more insights.
